RSC Advances

PAPER

Publ

ishe

d on

03

Sept

embe

r 20

14. D

ownl

oade

d by

Lib

rary

of

Chi

nese

Aca

dem

y of

Sci

ence

s on

02/

07/2

015

09:1

3:50

.

View Article OnlineView Journal | View Issue

Hg2+ ion-imprint2+

aKey Laboratory of Coastal Environmental

Shandong Provincial Key Laboratory of C

Institute of Coastal Zone Research, Chines

China. E-mail: lxchen@yic.ac.cn; Fax: +86 5bSchool of Chemistry & Chemical EngineerincKey Lab of Environmental Engineering in Sh

& Municipal Engineering, Qingdao TechnolodUniversity of Chinese Academy of Sciences,

† Electronic supplementary informationdithizone alone and dithizone with mecurves of Hg-IIPs/NIPs for Hg2+ and kinarea and other related data of Hg-IIPs/model parameters for Hg-IIPs/NIPs, anadsorption towards Hg-IIP from four kinet

Cite this: RSC Adv., 2014, 4, 46444

Received 5th August 2014Accepted 3rd September 2014

DOI: 10.1039/c4ra08163c

www.rsc.org/advances

46444 | RSC Adv., 2014, 4, 46444–464

ed polymers sorbents based ondithizone–Hg chelation for mercury speciationanalysis in environmental and biological samples†

Zhong Zhang,ad Jinhua Li,a Xingliang Song,b Jiping Mac and Lingxin Chen*a

For mercury speciation analysis in environmental and biological samples, novel Hg2+ ion-imprinted

polymers (IIPs) were synthesized by a sol–gel process using the chelating agent dithizone, and then

dithizone–Hg2+ chelate as a template and 3-aminopropyltriethoxysilane as a functional monomer,

followed by solid-phase extraction (SPE) and atomic fluorescence spectroscopy (AFS) detection. The

resultant Hg-IIPs offered high binding capacity, fast kinetics, and their adsorption processes followed a

Langmuir isotherm and pseudo-second-order kinetic models. The IIPs displayed excellent selectivity

toward Hg2+ over its organic forms and other metal ions with selectivity factors of 19–34, as well as high

anti-interference ability for Hg2+ confronting with common coexistent ions. Through 10 adsorption–

desorption cycles, the IIPs showed a good reusability with a relative standard deviation within 5%.

Moreover, because of the chelation of dithizone, the IIPs could readily discriminate Hg2+ from organic

mercury. Thus, mercury speciation analysis could be attained by using IIPs-SPE-AFS, presenting high

detectability of up to 0.015 mg L�1 for Hg2+ and 0.02 mg L�1 for organic mercury. This method was

validated by using two certified reference materials with very consistent results. Satisfactory recoveries

ranging from 93.0–105.2% were attained for spiked seawater and lake water samples with three

concentration levels of Hg2+. Furthermore, the analytical results for the spiked mercury species in real

biological samples, such as human hair and fish meat, confirmed that the methods are practically

applicable to speciation analysis. The IIPs-SPE-AFS demonstrated significant application perspectives for

rapid and high-effective cleanup, enrichment and determination of trace mercury species in complicated

matrices.

1. Introduction

Heavy metals have received wide international concern becauseof their difficult degradation and high bioaccumulation prop-erties, which results in environmental and ecotoxicologicalthreats.1–4 Among them, mercury (Hg) has become an attractiveresearch area1–6 because it has negative impacts on humansusually due to exposure through food, especially sh.7,8 Its

Processes and Ecological Remediation,

oastal Environmental Processes, Yantai

e Academy of Sciences, Yantai 264003,

35 2109130; Tel: +86 535 2109130

g, Linyi University, Linyi 276005, China

andong Province, School of Environment

gical University, Qingdao 266033, China

Beijing 100049, China

(ESI) available: The UV-vis spectra ofrcury species, the adsorption kineticsetics models data, the specic surfaceNIPs obtained by BET, the isothermd the parameters obtained of Hg2+

ic models. See DOI: 10.1039/c4ra08163c

53

toxicity depends on its chemical forms; organic mercurycompounds are generally much more toxic than inorganicmercury due to their strong lipophilic characteristics.7–10 Hence,speciation analysis of mercury ion (Hg2+) and its organic speciesis being increasingly investigated because the monitoring andremediation of heavy metal pollution has become a crucialglobal issue. The speciation and determination of mercuryspecies is generally performed using chromatographic tech-niques, such as gas chromatography (GC), liquid chromatog-raphy (LC), ion chromatography (IC) and capillaryelectrophoresis (CE) for separation,5,6,8,11 which are oencoupled with element selective detectors, e.g. inductivelycoupled plasma mass spectrometry (ICP-MS),12 atomic absorp-tion spectroscopy (AAS),13 and atomic uorescence spectroscopy(AFS).14–16 However, these instrumental techniques are usuallycomplex, costly, and time-consuming. On the other hand, thetrace/ultra-trace presence of targeted analytes in complicatedmatrices has become a bottleneck for actual application anal-ysis.17 Hence, it is important to develop high-efficiencypretreatment and enrichment techniques/processes. Amongthem, solid-phase extraction (SPE) with selective sorbents isgaining popularity.18,19

This journal is © The Royal Society of Chemistry 2014

Paper RSC Advances

Publ

ishe

d on

03

Sept

embe

r 20

14. D

ownl

oade

d by

Lib

rary

of

Chi

nese

Aca

dem

y of

Sci

ence

s on

02/

07/2

015

09:1

3:50

. View Article Online

Ion-imprinted polymers (IIPs), a branch of molecularlyimprinted polymers (MIPs), have attracted signicant interestas selective SPE sorbents for particular chemical forms of givenelements.4,20–24 In contrast to MIPs, IIPs recognize inorganicions aer imprinting, especially metal ions.20,21,24–27 Thisrecognition ability can be explained by the memory effects ofpolymers toward metal ion interaction with a specic ligand,coordination geometry, metal ion coordination number, chargeand size.28 Metal IIPs have attracted increasing attention due totheir unique features: rst, the coordination reaction is strongerthan the hydrogen bond strength, especially in aqueous solu-tion; second, it is quick to achieve the balance of the thermo-dynamics and kinetics.29,30 Lately, a number of studies on IIPsand their applications for selective preconcentration and sepa-ration of metal ions have been reported.20,21,28,30,31 However, IIPsrelated studies are oen limited by the challenges of incompleteion removal and low selectivity because many metal ions havethe same charges, similar ionic radius and properties.28,32,33 Toovercome these limitations effectively, a graing method on thesurface of polymer/silica gel beads and utilizing selectiveligands and/or functional monomers for ion imprinting havebeen developed.33–35 For example, Dakova et al. prepared Hg(II)layer-coated silica gel particles (Hg(II)-IIP) for the speciation anddetermination of mercury in wine.34 Yan et al. proposed IIPsmesoporous materials for Zn2+ and Cd2+ detection, offering analternative to immobilize small molecules.35 Chen et al.synthesized specic ligand 3-isocyanatopropyltriethoxysilaneand a specic functional monomer for specially imprintingHg2+, and attained highly selective preconcentration of Hg2+ inwater samples.33

Dithizone, a well-known traditional ligand that acts as achelating agent for metal ions, has excellent selectivity advan-tages and it has been increasingly used for mercury.36–38 Forinstance, Liu and his coworkers developed a dithizone-func-tionalized SPE procedure followed by HPLC-ICP-MS for mercuryin water samples.37 Talpur et al. used surfactant coated aluminamodied by dithizone for the preconcentration and determi-nation of trace amounts of mercury coupled with cold vaporAAS.38 On the other hand, sol–gel technology, an easy-to-handleand ecofriendly strategy, has ourished for the synthesis of IIPsand MIPs.33,39–41 However, to the best of our knowledge, no workcombining IIPs, dithizone–Hg2+ chelate and sol–gel for mercuryspeciation analysis has been reported.

Inspired by these studies, herein, we propose the synthesis ofnovel Hg2+ imprinted IIPs by sol–gel process, using dithizone–Hg2+ chelate as template and 3-aminopropyltriethoxysilane asfunctional monomer, for mercury speciation analysis in envi-ronmental and biological samples via selective SPE coupledwith AFS detection. The selectivity of Hg-IIPs for Hg2+ versusother interfering metal ions (Zn2+, Cd2+, Pb2+) and organicmercury species (MeHg+, EtHg+) was investigated. Two certiedreference materials were used for validation, and the developedmethod was successfully applied for Hg2+ determination inenvironmental water samples, as well as organic mercuryspecies determination in human hair and sh meat sampleswith satisfactory results.

This journal is © The Royal Society of Chemistry 2014

2. Experimental2.1. Reagents and materials

Standard solution of (60 mg L�1) mercury chloride (HgCl2) andmethylmercury (MeHg) chloride, as well as ethylmercury chlo-ride (EtHg) were purchased from the CRM/RM InformationCenter of China (Beijing, China). Dithizone, 3-amino-propyltriethoxysilane (APTES) and tetraethoxysilicane (TEOS)were obtained from Sigma-Aldrich (Shanghai, China). Otherreagents and materials were supplied by Sinopharm ChemicalReagent (Shanghai, China). All the reagents were at leastanalytical grade and were used directly without further puri-cation, unless otherwise specied. Throughout the experimentaqueous solutions work were prepared using doubly purieddeionized (DDI) water, which was produced by a Milli-Q Ultra-pure water system with the water outlet operating at 18.2 MU

(Millipore, MA, USA); all pH measurements were performedusing a pHs-3TC digital pH meter equipped with a combinedglass–calomel electrode (Shanghai, China).

2.2. Instrumentation

Atomic uorescence measurements of mercury species werecarried out with an AFS-3000 double-channel nondispersiveatomic uorescence spectrometer (Beijing Kechuang HaiguangInstrument Co., China). ICP-MS analyses were performed on aPerkinElmer Elan DRC II (USA). A FT-IR spectrometer (NicoletiS10, Thermo scientic) was employed to record the infraredspectra of samples using a pressed KBr tablet method. A scan-ning electron microscope (SEM, Hitachi S-4800, Japan) wasemployed to investigate the size and morphology of the poly-mers. Specic surface area of the polymers was determined byBrunauer–Emmett–Teller (BET) analysis by performingnitrogen adsorption experiments, and the measurement wasoperated on an AUTOSORB 1 (Quantachrome Instruments,Germany). The polymers were degassed in vacuum at 300 �Cprior to adsorption measurements. Zeta potential measure-ments were performed on a Malvern Zetasizer Nano-ZS90(ZEN3590, UK).

2.3. Preparation of Hg-IIPs

Hg-IIPs based on dithizone–Hg2+ interactions were prepared bya sol–gel process similar to a reported procedure33 with somemodications. Dithizone (256 mg) and HgCl2 (108 mg) weredissolved in a 100 mL ethanol solution to form a templatemolecules complex, followed by adding APTES (467 mL) asfunctional monomers, which were stored at 4 �C in dark for 6 h.Then, TEOS (2 mL), as crosslinking agent, and aqueousammonia (5 mL, 14%) were added in the mixing solution. Thepolymerization reaction was undertaken under magnetic stir-ring at room temperature for 12 h, and the product was furtheraged by stirring at 60 �C for 6 h to obtain high cross-linkingdensity and interconnected macroporous structures. Theresultant IIPs were obtained by centrifugation and then rinsedwith anhydrous ethanol for three times to remove the residues.Hg2+ was removed from the prepared IIPs by several sequentialelution steps with 0.5 mol L�1 HCl under vigorous stirring. The

RSC Adv., 2014, 4, 46444–46453 | 46445

RSC Advances Paper

Publ

ishe

d on

03

Sept

embe

r 20

14. D

ownl

oade

d by

Lib

rary

of

Chi

nese

Aca

dem

y of

Sci

ence

s on

02/

07/2

015

09:1

3:50

. View Article Online

nal products were washed with DDI water up to pH 6–7 andwere dried under vacuum at 40 �C. For simplicity, the obtainedpolymers were named Hg-IIPs. For comparison, the non-imprinted polymers (NIPs) as control materials were preparedusing the same procedure and conditions, only in the absenceof the template molecules.

2.4. Batch procedure

To evaluate the ion recognition properties of the Hg-IIPs, theeffect of pH in the solution, static and dynamic adsorption andselectivity and reusability were tested by a batch proceduremethod. The procedures were carried out as follows. The Hg-IIPs (20 mg) were equilibrated with 10 mL of aqueous solutionscontaining 20 mg L�1 of Hg2+ with pH varying from 2 to 9(adjusted using 0.1 mol L�1 of phosphate buffer) at roomtemperature for 12 h. The pH was maintained in a range of�0.1units. The tested solutions were centrifuged and the superna-tant solutions were collected and ltered through a 0.45 mmmembrane. Finally, the ltrate solutions were characterized byAFS. The instrumental response was periodically tested withknown metal standard solutions. The experiments were per-formed in triplicates.

Static adsorption was examined using 10 mL aqueous solu-tions containing Hg2+ at various concentrations, i.e. from 2 to 20mg L�1, incubating for 12 h under optimal sorption conditions.The binding amount (Q) of Hg2+ was calculated by subtractingthe free concentrations from the initial concentrations. Mean-while, dynamic adsorption test was carried out by monitoringthe temporal amount of Hg2+ in the solutions with the initialconcentration of 10 mg L�1. The mixture was continuouslyshaken in a thermostatically controlled water bath at roomtemperature for 0 to 160 min. The polymers were removed bycentrifugation and the supernatant solutions were collectedand determined using AFS, which was similar to that of thestatic adsorption test.

The selectivity adsorption experiments were performed byusing Zn2+, Cd2+, Pb2+, MeHg+ and EtHg+ at the same concen-tration as Hg2+ (4 mg L�1), which were then treated with the Hg-IIPs and NIPs. The supernatant solutions were collected anddetermined using ICP-MS. Interference tests were conducted byusing MeHg+, EtHg+, Na+, K+, Zn2+,Cu2+, Pb2+, Mg2+, Ca2+, Mg2+,Cd2+ and Fe3+ as co-existent ions at concentrations 10 timeshigher than of Hg2+ (40 mg L�1). The experimental processeswere similar to the abovementioned static adsorption test withthe same polymer mass of 20mg. All the tests were performed intriplicates. The average data from triplicate independent resultswere used for the following discussion.

In addition, the recognition ability of the Hg-IIPs was eval-uated by using the imprinting factor (a), which is dened as:

a ¼ QIIP/QNIP (1)

where, QIIP and QNIP are the adsorption amounts of template oranalogues on IIPs and NIPs at equilibrium, respectively.

The reusability of Hg-IIPs was assessed by the adsorption–desorption experiments. Briey, the Hg-IIPs were rst put inHg2+ solution (4 mg L�1) for saturated adsorption, and then the

46446 | RSC Adv., 2014, 4, 46444–46453

adsorbed Hg2+ ions could be desorbed from the Hg-IIPs byusing a 0.5 mol L�1 HCl solution as desorption medium, aervigorously stirring for 2 h at room temperature. The nal Hg2+

in the aqueous solution was detected by AFS. The adsorption–desorption studies were repeated ten times using the sameimprinted polymers.

2.5. Standard solution and practical sample preparation

The stock standard solutions of MeHg+ at 1000 mg L�1 wereobtained by dissolving appropriate amounts of methylmercurychloride in HPLC grade methanol. Lake water samples werecollected in a teon bottle from an articial lake located inLaishan District of Yantai City. Surface seawater samples werecollected in a teon bottle from the Fisherman's Wharf of theYellow Sea located in the coastal zone area of Yantai City. All thewater samples were ltered through 0.45 mm PTFE syringelters (Phenomenex, Los Angeles, CA, USA) to remove sus-pended particles. The resultant ltrates, which were directlyanalyzed or alkalied to pH 7.0 using 0.1 mol L�1 NaOH, werekept in a refrigerator at 4 �C prior to use.

Two certied reference materials (CRMs), namely freeze-dried sh powder (BCR-463) and human hair (GBW07601(GSH-1)), purchased from the CRM/RM Information Center of China(Beijing, China), were used to validate the accuracy and preci-sion of the developed method. Moreover, for practical sampletests, human hair samples were randomly collected fromhealthy volunteers in our lab and sh meat was purchased froma local supermarket. The sampled were treated according toprevious reports,42 as follows: 1.0 g samples were added to acentrifuge tube containing 1.4 mL of 10 mol L�1 NaOH. Thetube was heated at 90–95 �C for 30 min. Then, the solution wascooled to room temperature and its pH was adjusted to 7.0using concentrated hydrochloric acid, followed by centrifuga-tion to obtain the supernatant. The treatment procedure wasrepeated three times, and all the three portions of supernatantswere merged for AFS detection.

2.6. SPE procedure

A PTFE column (Phenomenex, Los Angeles, CA, USA) waspacked with 200 mg of Hg-IIPs using a syringe with 4.0 mm i.d.The column was preconditioned successively with 5 mL of 0.1mol L�1 HCl, 5 mL of DDI water and 5 mL of blank solutions(standard or sample solutions without spiking). Then, 50 mL ofthe sample solution containing Hg2+ at three concentrations (2,5 and 10 mg L�1) was passed through the column at a ow rate of1.0 mLmin�1. The column was washed with 10 mL of DDI waterand the adsorbed Hg2+ was eluted with 2.5 mL of 0.5 mol L�1

HCl. Then, the extractants were detected by AFS. Each concen-tration was analysed using three replicates.

3. Results and discussion3.1. Preparation of Hg2+ imprinted IIPs

The synthesis and imprinting process of Hg2+ imprinted IIPs viathe sol–gel process is schematically shown in Fig. 1. First, theHg2+ and dithizone complex was produced based on the strong

This journal is © The Royal Society of Chemistry 2014

Fig. 2 (Upper) FT-IR spectra of (a) Hg-NIPs, (b) Hg-IIPs and (c) Hgcontained Hg-IIPs. (Below) SEM images of Hg-IIPs.

Paper RSC Advances

Publ

ishe

d on

03

Sept

embe

r 20

14. D

ownl

oade

d by

Lib

rary

of

Chi

nese

Aca

dem

y of

Sci

ence

s on

02/

07/2

015

09:1

3:50

. View Article Online

chelation of dithizone to Hg2+ ions, which was used as templatemolecules. The template molecules could easily pre-polymerizewith the functional monomers APTES by non-covalent interac-tions. APTES, a commonly used silane coupling agent for sol–gel process, was employed due to the hydrophobic interaction.Then, the polymerizable group of APTES copolymerized withTEOS. Therefore, APTES played an important role in the prep-aration of Hg-IIPs. Finally, Hg2+ was removed by elution, leavingbehind specic recognition sites on the surface of the polymers,with functional groups in a predetermined orientation andproper size cavities for Hg2+ ions, which contributes to the highadsorption efficiency and selectivity for Hg2+ ions due to metal–ligand chemistry.43 Meanwhile, the adopted sol–gel process,with distinct characteristics such as easy fabrication, usingecofriendly reaction solvents, and mild polymerization condi-tions,33 allowed dithizone–Hg2+ chelate, as template, to bereadily embedded in the highly cross-linked host structurewithout the problem of thermal or chemical decomposition.

Moreover, it was found that dithizone chelated Hg2+ but notorganic mercury compounds (Fig. S1†). Thus, it is possible torealize the speciation analysis of mercury species using the Hg-IIPs based on the specic, strong chelation of dithizone.Meanwhile, AFS can only measure total mercury with themercury hollow cathode lamp without discriminating betweenmercury species. Therefore, the prepared Hg-IIPs coupled withAFS would not only provide a superior selectivity for Hg2+ butalso a possible speciation analysis.

3.2. Characterization of the Hg-IIPs

To conrm the presence of dithizone in the silica gel sorbentsand its chelation for imprinting, the IR spectra of IIPs, Hg-contained IIPs and NIPs were measured. As shown in Fig. 2(upper), the wide and strong absorption band at around 1089and 945 cm�1 could be attributed to Si–O–Si and Si–O–Hstretching vibrations, respectively, and the absorption peaks ataround 799 and 467 cm�1 belonged to Si–O vibrations, indi-cating the presence of silica matrices in the three materials. Thestrong and broad absorption band observed at 3425 cm�1 could

Fig. 1 Schematic illustration of the preparation and imprinting processof Hg-IIPs.

This journal is © The Royal Society of Chemistry 2014

be assigned to the N–H stretching vibration of APTES.Compared with NIPs, the characteristic absorption peak at 1625cm�1 (Fig. 2a), which most probably represents the stretchingvibrations of N]N double bonds of the pyridine ring in dithi-zone, became signicantly weak in the IIPs (Fig. 2b) and Hg-contained IIPs (Fig. 2c). Interestingly, a new peak appeared at1607 cm�1 in Fig. 2c, which is just the characteristic peak of Hg–N bond. These observations demonstrated the coordination ofdithizone through the N atom from its fragment, indicating thatthe metal–ligand bond was formed between Hg2+ and thechelating groups of IIPs. The FT-IR results conrmed that theHg-IIPs were successfully prepared based on the metal–ligandchemistry by sol–gel polymerization.

SEM images were taken to characterize the surfacemorphologies of the Hg-IIPs. As observed, the IIPs displayed adendritic structure and appeared to be covered with particles inseveral areas (Fig. 2, below). The particles were likely to beaggregated silicon particles because of the use of APTES in thesol–gel process. Because the imprinting polymerization processoccurred, the polymers demonstrated a dendritic structure.However, in the absence of imprinting process, silicon particlesaggregation materials could be easily formed. Thus, herein, thedendritic-structured IIPs were reasonable with partial coverageof aggregated silicon particles. In addition, by BET analysis, aspecic surface area of 43.27 m2 g�1 was attained for Hg-IIPs,which is higher than that of the corresponding NIPs (22.92 m2

g�1). Fig. 3A shows N2 adsorption–desorption isotherms of theHg-IIPs and the corresponding NIPs. As seen, when the relativepressure P/P0 was less than 0.8, the slope of the curves was

RSC Adv., 2014, 4, 46444–46453 | 46447

Fig. 3 (A) N2 adsorption–desorption isotherms and (B) pore sizedistribution of IIPs and NIPs.

RSC Advances Paper

Publ

ishe

d on

03

Sept

embe

r 20

14. D

ownl

oade

d by

Lib

rary

of

Chi

nese

Aca

dem

y of

Sci

ence

s on

02/

07/2

015

09:1

3:50

. View Article Online

small, indicating that little amount of small pores were presenton the surface of IIPs. However, when the relative pressure P/P0was higher than 0.8, the slope of the curves signicantlyincreased. As seen in the gure, the desorption curve was closerbut leveled slightly above the adsorption curve, which could bethe evidence of a small quantity of micropores on the IIPs.44 Asshown in Fig. 3B, the average pore size for IIPs and NIPs was9.42 and 5.62 nm, respectively. The narrow pore diameterdistribution and the low average pore diameter suggested thatthe size of the cavities formed in the IIPs played a key role inbinding capacity. Related morphological structure parametersof IIPs and NIPs are listed in Table S1.† The results indicate thatthe IIPs have uniform and regular structure. The large cumu-lative pore volume was very likely because of the microspores onthe surface of IIPs, which revealed that the Hg2+ ions werealmost completely eluted and removed, and thereby leading toconsiderable amounts of imprinting cavity sites.

Fig. 4 Effect of pH on the adsorption of Hg2+ for Hg-IIPs. (Inset) Effectof pH on the Zeta potential of Hg-IIPs. Experimental conditions: IIPs,20 mg; C0, 10 mg L�1; V, 10 mL; adsorption time, 12 h.

3.3. Binding properties of the IIPs for Hg2+

Solution acidity plays an important role in the adsorptioncapacity because it generally affects the complexing reactionbetween metal ions and ligands. To evaluate the effect of

46448 | RSC Adv., 2014, 4, 46444–46453

varying pH on Hg2+ adsorption, a batch procedure was used. Asseen in Fig. 4, the adsorption capacity was very low at pH valuesbelow 4.0. The N atoms of dithizone are highly protonated atlow pH, and the electronic-supply ability of N atoms signi-cantly weakens, thereby reducing the chelating effect of dithi-zone for Hg2+ ions,31,32 which in turn reduces the adsorptioncapacity of Hg-IIPs. The adsorption capacity increased rapidlyalong with an increase in pH to 4–6 because the protonation ofN atoms of dithizone became weak and enhanced the adsorp-tion capacity of dithizone for Hg2+. Above pH 6.0, the increasewas relatively slow, and adsorption capacity approached amaximum at pH 8.0. At pH > 9, the formation and precipitationof metal hydroxide, as well as quick decomposition, easilyoccurs. At the same time, the IIPs would become electronega-tive, enabling them to adsorb various other metal ions to acertain extent, thus causing a loss of selectivity. In addition, ascan be seen from the inset of Fig. 4, Zeta potential of Hg-IIPsvaried in different pH values with an excellent linearity in therange of pH 2–9. Thus, the adsorption capacity of IIPs for Hg2+

was dependent on pH. The optimum pH for the adsorption ofHg2+ from aqueous solutions ranged from 7.0 to 8.0. Within thisrange, neither the precipitation of the metal hydroxide nor theprotonation of the N atoms were obvious. Considering theirpossible applications in physiological conditions, pH 7.0 wasselected to be optimum for further experiments.

Then, static adsorption experiments were investigated toevaluate the adsorption capacities of Hg-IIPs for Hg2+ within 2–20 mg L�1 at pH 7.0. As shown in Fig. 5A, the amounts of Hg2+

adsorbed per unit mass of IIPs increased along with an increasein the initial concentrations of Hg2+, and leveled off aer 16 mgL�1. Excitingly, the experimentally obtained maximum adsorp-tion capacity of Hg-IIPs was about 10 times than that of NIPs,presenting the imprinting factor a of 9.58. This result suggeststhat the IIPs possess high imprinting effect due to the highbinding affinity for Hg2+. Furthermore, the obtained staticcapacity data for IIPs were tted using Langmuir, Freundlich

This journal is © The Royal Society of Chemistry 2014

Table 1 Selectivity of the prepared Hg-IIPs and NIPs

Metal ions

Hg-IIPs NIPs

Da (mL g�1) Sb D (mL g�1) S

Hg2+ 452.8 — 47.3 —MeHg+ 23.8 19.06 28.1 1.67EtHg+ 21.1 21.56 26.2 1.82Zn2+ 13.5 33.54 15.3 3.10Cd2+ 16.9 26.63 21.8 2.17Pb2+ 20.5 22.09 23.3 2.03

a Distribution ratio, D ¼ Q/Ce. b Selective coefficient, S ¼ Dtemplate/Dcompetetive ion. Dtemplate and Dcompetetive ion are distribution ratio of thetemplate and the competitive ions on IIPs or NIPs, respectively.

Fig. 5 (A) Static adsorption isotherm curves of IIPs and NIPs for Hg2+

in aqueous solutions, and (B) comparison of Langmuir, Freundlich andLangmuir–Freundlich isotherm models for Hg2+ adsorption processonto the IIPs. Experimental conditions: V, 10 mL; polymer, 20 mg;adsorption time, 12 h; room temperature.

Paper RSC Advances

Publ

ishe

d on

03

Sept

embe

r 20

14. D

ownl

oade

d by

Lib

rary

of

Chi

nese

Aca

dem

y of

Sci

ence

s on

02/

07/2

015

09:1

3:50

. View Article Online

and Langmuir–Freundlich isotherm models. As shown inFig. 5B, the Langmuir isotherm model yielded the best t,showing the highest correlation coefficient of R2 ¼ 0.998 (TableS2†). Therefore, the data obtained from the equilibrium studiesfor the adsorption of Hg2+ ions onto Hg-IIPs may follow theLangmuir model, which assumes that adsorption occurs atspecic homogeneous sites on the adsorbent and can besuccessfully applied to monolayer adsorption processes.45 Thisindicates that Hg2+ ions are adsorbed as a monolayer onto thesurface of Hg-IIPs. In addition, the Hg-IIPs present the highestconcentration of binding sites per gram of polymers and thelargest median binding affinity (Table S2†), further conrmingtheir excellent imprinting effect due to a number of specicbinding sites. Dynamic binding experiments were carried out toevaluate the binding rate and ion transfer properties of Hg2+

onto the IIPs. As shown in Fig. S2A,† 80 min were required toreach an adsorption equilibrium for Hg-IIPs. In the sameadsorption time, the binding capacity of Hg-IIPs was

This journal is © The Royal Society of Chemistry 2014

remarkably higher than that of NIPs, indicating that rebindingviametal–ligand bond favors ion transfer and binding capacity.Furthermore, the dynamic binding was investigated by differentmodels, including pseudo-rst-order, pseudo-second-order,Elovich and intra-particle diffusion.46 Showing the highestcorrelation coefficient of R2 ¼ 0.999 (Table S3†), the pseudo-second-order model provided the most suitable correlation forthe Hg-IIPs adsorption, which can be expressed as follows:

t

Qt

¼ 1

k2Qe2þ t

Qe

(2)

where Qt is the instantaneous binding capacity at time t, Qe isthe equilibrium binding capacity, and k2 is the rate constant.The obtainedQe of 3.74 mmol g�1 calculated from themodel wasin good agreement with the Qe of 3.68 mmol g�1 obtained fromexperimental results. In addition, as shown in Fig. S2B,† thecurve showing the entire time period was found to be the bestpredicting the kinetic process. Therefore, it can be deduced thatadsorption follows the pseudo-second-order kinetics model.Furthermore, it could be expected that the rate-limiting stepmight be chemisorption involving valency forces through thesharing or exchange of electrons between sorbent and sorbate.47

Therefore, chemisorption could be the rate-limiting step in theadsorption process of IIPs for Hg2+, further conrming that theHg-IIPs were prepared based on metal–ligand chemistry.

3.4. Ionic selectivity and recognition reliability of the IIPs

High ionic selectivity and recognition reliability are recognizedas the chief requirements and features of IIPs. Competitiveadsorption from binary mixtures, including Hg2+/MeHg+, Hg2+/EtHg+, Hg2+/Zn2+, Hg2+/Cd2+ and Hg2+/Pb2+, was investigated inselective binding experiments because these ions oen coexistin practical samples. As can be seen in Table 1, the competitiveadsorption capacity of Hg-IIPs for Hg2+ is considerably higherthan that of NIPs. The selective coefficients of Hg-IIPs for binarymixtures were obtained in the range of 19–34, which is likelydue to the highly selective Hg2+–dithizone interactions. Inter-estingly, this implied that the determination of organic mercurycan be realized using the IIPs by subtracting the amounts ofHg2+ from total mercury.

RSC Adv., 2014, 4, 46444–46453 | 46449

Fig. 6 Adsorption capacities for Hg2+ in the presence of 4 mg L�1

Hg2+ and 40mg L�1 of other metal ions, including MeHg+, EtHg+, Na+,K+, Zn2+, Cu2+, Pb2+, Mg2+, Ca2+, Mg2+, Cd2+ and Fe3+. Experimentalconditions: IIPs, 20 mg; V, 10 mL; pH, 7.0.

RSC Advances Paper

Publ

ishe

d on

03

Sept

embe

r 20

14. D

ownl

oade

d by

Lib

rary

of

Chi

nese

Aca

dem

y of

Sci

ence

s on

02/

07/2

015

09:1

3:50

. View Article Online

To further evaluate the anti-interference ability and reli-ability of the IIPs for Hg2+, the mixture solutions containingHg2+ and various possible interfering ions were examined withconcentrations of interfering ion 10 times higher than Hg2+ inevery case. As shown in Fig. 6, those ions did not cause signif-icant interference without remarkable reduction in the capacityfor Hg2+ determination aer the IIPs preconcentration proce-dure. It is possible to estimate that only <10% of the bindingsites were taken over by the interfering ions, even when theirconcentration was 10 times higher. All the results indicate thatthe Hg-IIPs are highly selective and reliable for Hg2+

recognition.

3.5. Reusability of the IIPs

As an important index, the reusability of the IIPs is also inves-tigated because of their pivotal role in cost-saving. Desorptionof Hg2+ from the IIPs were studied by using 0.5 mol L�1 of HCl,and 0.5 mol L�1 of HCl plus 0.1 mol L�1 thiourea. It was foundthat the rebinding capacity was signicantly reduced whenusing the latter as the desorptionmedium, which was very likelydue to the disruption of the non-covalent interaction betweenfunctional monomers and dithizone, and thereby the complexof dithizone and Hg2+ was desorbed. In contrast, aer treatment

Table 2 Determination of MeHg+ and total Hg in certified reference ma

CRMs

Certied (mg g�1) Determ

MeHg+ Total Hg MeHg+

BCR-463 2.85 � 0.16 3.04 � 0.16 2.95 �GBW07601 — 0.36 � 0.08 NDe

a The comparison of the developed Hg-IIPs-SPE-AFS and ICP-MS results wfrom this method using AFS. c Results obtained by ICP-MS. d Average valu

46450 | RSC Adv., 2014, 4, 46444–46453

with 0.5 mol L�1 of HCl, the IIPs displayed high rebindingcapacity without obvious decrease in activity. It could be infer-red that the chelation of dithizone for Hg2+ was destroyed andHg2+ ions were subsequently released into the desorptionmedium, which satised the requirements of IIPs. Therefore,0.5 mol L�1 of HCl was utilized as an ideal desorption mediumfor the repeated 10 adsorption–desorption cycles. Finally, theHg-IIPs still presented high rebinding capacities for Hg2+ withonly a slight decrease within 5%. Consequently, the Hg-IIPspresent excellent reusability and regeneration ability, whichfavors actual applications.

3.6. Analytical performances for Hg2+ and organic mercurydetermination by using IIPs-SPE

Based on the abovementioned results, the Hg-IIPs could beemployed as SPE sorbents for the preconcentration of Hg2+. Theanalytical performances of Hg-IIPs-SPE coupled with AFS forHg2+ and total Hg determination were investigated. Under theoptimized SPE conditions, an excellent linearity from 0.05 to 15mg L�1 for Hg2+ was observed, and the relative standard devia-tion (RSD) at 2 mg L�1 was 5.2%. The limit of detection (LOD)based on three times the standard deviation and for 10 replicatemeasurements of a blank solution was 0.015 mg L�1, which issignicantly lower than the mandated upper limit of 2 mg L�1

for Hg2+ in drinking water by the United States EnvironmentalProtection Agency (USEPA). This indicates that the Hg-IIPs-SPE-AFS are potentially applicable for the sensitive and accuratedetermination of Hg2+ in drinking water.

Using the developed method, the organic mercury valuecould be obtained by taking the total Hg value and subtractingthe Hg2+ value, thereby the organic mercury species could bedetermined. The LOD achieved for MeHg+ was 0.02 mg L�1,higher than the permitted maximum concentration level (MCL)of 0.001 mg L�1 by China. Nevertheless, the method sensitivitymight contribute to mercury speciation analysis in watersamples, especially industrial and sanitary wastewater. TheLOD for EtHg+ was 0.02 mg L�1, lower than theMCL of 0.1 mg L�1

formulated by China. In addition, the RSDs at 2 mg L�1 were in arange of 6–11%. Hence, the Hg-IIPs-SPE-AFS could recognizeand detect organic mercury. Thus, this method clearly proves tobe potentially feasible to quantitatively analyze organic mercuryin real water samples.

To validate this established method, the Hg-IIP-SPE proce-dure was applied to the CRMs, BCR-463 (sh meet) and

terials (CRMs)a

ined (mg g�1)b Determined (mg g�1)c

Total Hg MeHg+ Total Hg

015d 3.21 � 0.15 2.89 � 0.14 3.15 � 0.140.38 � 0.07 ND 0.36 � 0.06

as made by using the t-test at 95% condence level. b Results obtainede of three determinations � standard deviation. e Not detected.

This journal is © The Royal Society of Chemistry 2014

Paper RSC Advances

Publ

ishe

d on

03

Sept

embe

r 20

14. D

ownl

oade

d by

Lib

rary

of

Chi

nese

Aca

dem

y of

Sci

ence

s on

02/

07/2

015

09:1

3:50

. View Article Online

GBW07601 (human hair). As shown in Table 2, the determinedvalues for MeHg and total Hg were 2.95� 0.15 mg g�1 and 3.21�0.15 mg g�1, respectively; in BCR-463, the total Hg was 0.38 �0.07 mg g�1 in GBW07601, which agrees well with the certiedvalues. The t-test conrmed that there was no signicantdifference between the developed method and the ICP-MSdetermination at 95% condence level. Therefore, the Hg-IIPs-SPE-AFS was proven to be accurate and reliable for mercuryspeciation in real solid and semi-solid samples.

3.7. Applications of the IIPs-SPE to environmental andbiological samples

To evaluate the practical applicability of the developed methodfor the mercury speciation analysis, the IIPs-SPE was applied inenvironmental water samples and biological samples. As shownin Table 3, recoveries were in the range of 93.0–105.2% withRSD of 2.9–5.3%, suggesting that the obtained IIPs, via thechelating interaction of dithizone and Hg2+ accompanied withdual functional monomers, were ideal candidates for SPEsorbents and were applicable for the effective enrichment andquantitative determination of trace Hg2+ in real water samples.At the same time, it suggested that the matrix effects weresharply reduced aer the IIPs-SPE procedure. In addition, the t-test conrmed that the determined data was in good consis-tency with that obtained by ICP-MS (Table 3). Hence, themethod was feasible for real sample analysis with a high

Table 4 Analytical results of the determination of Hg2+, MeHg+ and EtH

Sample

Added (mg L�1) Found (m

MeHg+ EtHg+ Hg2+ Organic m

Human hair 0 0 10 ND10 0 10 10.45b

0 10 10 9.78

Fish meat 0 0 10 ND10 0 10 9.620 10 10 9.78

a Organic mercury value was obtained subtracting the Hg2+ value to the t

Table 3 Analytical results of the determination of seawater and lake wa

Sample Added (mg L�1) Found (mg L�1

Seawater 0 0.212 2.155 5.47

10 9.76

Lake water 0 0.312 2.245 4.96

10 10.74

a Average value from ve individual experiments. b Relative standard devi

This journal is © The Royal Society of Chemistry 2014

accuracy and good reliability. Moreover, the endogenouscontents of Hg2+ were detected at 0.21 and 0.31 mg L�1 in thetested seawater and lake water samples, respectively. This valueis signicantly lower than the permitted concentration of Hg2+

in drinking water by USEPA (2.0 mg L�1). Therefore, the vali-dated IIPs-SPE coupled to AFS method might be an idealalternative to the simultaneous enrichment, separation anddetermination of Hg2+ in complicated water samples for envi-ronmental monitoring and remediation.

In addition, the practicality of the IIPs-SPE for organicmercury in biological samples was investigated. As observed inTable 4, recoveries for organic mercury ranged from 97.8–104.5% with RSDs of 7.9–8.1% for human hair and 96.2–97.8%with RSDs of 8.6–9.3% for sh meat samples. Therefore, theIIPs-SPE-AFS was demonstrated to be capable of simply andsensitively identifying and detecting organic mercury species.

3.8. Comparison with other methods

Performance of this developed chelate imprinting strategytoward mercury species was compared with that of somereported IIPs-SPE based methods. As shown in Table S4,† as anew material, the prepared IIPs are ideal sorbent candidates inSPE for the preconcentration of Hg2+. The attained LOD is loweror comparable to the reported values.33,34,48 Species includingHg2+, MeHg+ and EtHg+ are analyzed. The presented IIPs aresimpler and easier to prepare using a cheaper classic ligand

g+ in biological samples

g L�1) Recovery � RSD (%)

ercurya Total Hg Organic mercury Total Hg

9.85 — 98.5 � 2.421.23 104.5 � 8.1 106.2 � 4.218.52 97.8 � 7.9 92.6 � 3.4

10.24 — 102.4 � 4.118.76 96.2 � 9.3 93.8 � 5.319.75 97.8 � 8.6 98.8 � 3.7

otal Hg value. b Average value from three individual experiments.

ter samples spiked with Hg2+

) Recoverya � RSDb (%)ICP-MS results(mg L�1)

— 0.2397.0 � 5.3 2.05

105.2 � 4.7 5.0795.5 � 3.2 9.84

— 0.2995.5 � 4.5 2.0693.0 � 2.9 5.09

104.4 � 3.6 10.13

ation.

RSC Adv., 2014, 4, 46444–46453 | 46451

RSC Advances Paper

Publ

ishe

d on

03

Sept

embe

r 20

14. D

ownl

oade

d by

Lib

rary

of

Chi

nese

Aca

dem

y of

Sci

ence

s on

02/

07/2

015

09:1

3:50

. View Article Online

(dithizone) than 1-pyrrolidinedithiocarboxylic acid34 and diaz-oaminobenzene48 by a milder sol–gel process, as well as avoid-ing synthesizing procedures.33 Moreover, they are applicable inmore samples, including environmental and biologicalsamples.

4. Conclusions

In summary, the novel Hg2+ IIPs based on dithizone–Hg2+

chelation were successfully prepared and applied to environ-mental and biological samples for the preconcentration of Hg2+

and mercury speciation analysis by a facile sol–gel process.Using dithizone–Hg2+ complex as a template, the easilyobtained IIPs displayed fast adsorption kinetics and highbinding capacity for Hg2+, excellent selectivity towards Hg2+ overorganic mercury and other metal ions, as well as high anti-interference ability for Hg2+ recognition and adsorption.Coupled to AFS detection, a high detection ability of up to 0.015mg L�1 for Hg2+ and 0.02 mg L�1 for organic mercury could beachieved, comparable to or even surpassing that afforded bytraditional chromatography-mass spectroscopy methods. Themethod accuracy was validated with consistent results forCRMs. In addition, the applicability was demonstrated for theanalysis of Hg2+ in seawater and lake water samples, togetherwith organic mercury species in human hair and sh meatsamples. Therefore, the IIPs-SPE-AFS proved to be a verypromising, sensitive, simple and ecofriendly alternative toconventional methods for mercury speciation analysis inaqueous, solid and semi-solid environmental and biologicalsamples.

On the other hand, we plan to develop a simple generalapplicable IIPs based platform for heavy metal ions and speciesby means of the model of Hg2+ imprinting. For example, MeHg+

can also be imprinted by using specic ligands and/or func-tional monomers. With a suitable choice and reasonable utili-zation of chelation, traditional ligands and functionalmonomers (especially those commercially available), and bysmartly devising and synthesizing new ligands and functionalmonomers, the IIPs-based heavy metal detection/removal plat-form can be developed for the selective, sensitive routinemonitoring/remediation of environmental and biologicalsamples. Such an excellent platform has signicant potentialfor imprinting various heavy metals, which certainly will pushthe development of heavy metal speciation analysis and enrichthe research of molecular imprinting technologies. We believethat it will provide new opportunities to explore novel IIPs/MIPsfor potential utilizations. Our efforts in this regards arecurrently undergoing.

Acknowledgements

This work was nancially supported by the National NaturalScience Foundation of China (21275158, 21105117, 21107057,21275068), the Scientic Research Foundation for the ReturnedOverseas Chinese Scholars, State Education Ministry, and theInnovation Projects of the Chinese Academy of Sciences(KZCX2-EW-206).

46452 | RSC Adv., 2014, 4, 46444–46453

Notes and references

1 Q. H. Wu, N. F. Y. Tam, J. Y. S. Leung, X. Z. Zhou, J. Fu,B. Yao, X. X. Huang and L. H. Xia, Ecotoxicol. Environ. Saf.,2014, 104, 143.

2 G. Aragay, J. Pons and A. Merkoc, Chem. Rev., 2011, 111,3433.

3 A. R. Timerbaev, Chem. Rev., 2013, 113, 778.4 Y. G. Yin, J. F. Liu and G. B. Jiang, Chin. Sci. Bull., 2013, 58,150.

5 Z. F. Liu, Z. L. Zhu, H. T. Zheng and S. H. Hu, Anal. Chem.,2012, 84, 10170.

6 J. H. Li, W. H. Lu, J. P. Ma and L. X. Chen, Microchim. Acta,2011, 175, 301.

7 L. H. Reyes, G. M. Mizanur Rahman, T. Fahrenholz andH. M. Kingston, Anal. Bioanal. Chem., 2008, 390, 2123.

8 F. A. Duarte, B. M. Soares, A. A. Vieira, E. R. Pereira,J. V. Maciel, S. S. Caldas and E. G. Primel, Anal. Chem.,2013, 85, 5015.

9 J. B. Shi, L. N. Liang, G. B. Jiang and X. L. Jin, Environ. Int.,2005, 31, 357.

10 D. Karunasagar, M. V. Balarama Krishna, S. V. Rao andJ. Arunachalam, J. Hazard. Mater., 2005, 118, 133.

11 F. F. Yang, J. H. Li, W. H. Lu, Y. Y. Wen, X. Q. Cai, J. M. You,J. P. Ma, Y. J. Ding and L. X. Chen, Electrophoresis, 2014, 35,474.

12 D. Yan, L. M. Yang and Q. Q. Wang, Anal. Chem., 2008, 80,6104.

13 Y. Li, Y. Jiang and X. P. Yan, Anal. Chem., 2006, 78, 6115.14 T. Yordanova, I. Dakova, K. Balashev and I. Karadjova,

Microchem. J., 2014, 113, 42.15 J. Margetınova, P. H. Pelcova and V. Kuban, Anal. Chim. Acta,

2008, 615, 115.16 W. B. Zhang, X. A. Yang, Y. P. Dong and J. J. Xue, Anal. Chem.,

2012, 84, 9199.17 Y. Y. Wen, J. H. Li, J. P. Ma and L. X. Chen, Electrophoresis,

2012, 33, 2933.18 W. A. W. Ibrahim, L. I. Abd Ali, A. Sulaiman, M. M. Sanagi

and H. Y. Aboul-Enein, Anal. Chem., 2014, 44, 233.19 Y. Y. Wen, L. Chen, J. H. Li, D. Y. Liu and L. X. Chen, TrAC,

Trends Anal. Chem., 2014, 59, 26.20 L. X. Chen, S. F. Xu and J. H. Li, Chem. Soc. Rev., 2011, 40,

2922.21 X. Q. Cai, J. H. Li, Z. Zhang, F. F. Yang, R. C. Dong and

L. X. Chen, ACS Appl. Mater. Interfaces, 2014, 6, 305.22 N. T. Hoai, D. K. Yoo and D. Kim, J. Hazard. Mater., 2010,

173, 462.23 C. R. T. Tarley, F. N. Andradea, F. M. Oliveirac,

M. Z. Corazzac, L. F. M. Azevedoa and M. G. Segatelli, Anal.Chim. Acta, 2011, 703, 145.

24 S. L. Wei, Y. Liu, M. D. Shao, L. Liu, H. W. Wang andY. Q. Liu, RSC Adv., 2014, 4, 29715.

25 N. Zhang and B. Hu, Anal. Chim. Acta, 2012, 723, 54.26 A. Bhaskarapillai and S. V. Narasimhan, RSC Adv., 2013, 3,

13178.27 B. B. Prasad, D. Jauhari and A. Verma, Talanta, 2014, 20, 398.

This journal is © The Royal Society of Chemistry 2014

Paper RSC Advances

Publ

ishe

d on

03

Sept

embe

r 20

14. D

ownl

oade

d by

Lib

rary

of

Chi

nese

Aca

dem

y of

Sci

ence

s on

02/

07/2

015

09:1

3:50

. View Article Online

28 C. Branger, W. Meouche and A. Margaillan, React. Funct.Polym., 2013, 73, 859.

29 C. R. Preetha, J. M. G. Ladis and T. P. Rao, Environ. Sci.Technol., 2006, 40, 3070.

30 T. Alizadeh, M. R. Ganjali and M. Zare, Anal. Chim. Acta,2011, 689, 52.

31 B. BaliPrasad, D. Jauhari and A. A. Verma, Talanta, 2014, 120,398.

32 S. Clemens, M. Monperrus, O. F. X. Donard, D. Amourouxand T. Guerin, Talanta, 2012, 89, 12.

33 S. F. Xu, L. X. Chen, J. H. Li, Y. F. Guan and H. Z. Lu, J.Hazard. Mater., 2012, 237–238, 347.

34 I. Dakova, T. Yordanova and I. Karadjova, J. Hazard. Mater.,2012, 231, 49.

35 J. Tan, H. F. Wang and X. P. Yan, Biosens. Bioelectron., 2009,24, 3316.

36 K. Leopold, M. Foulkes and P. Worsfold, Anal. Chim. Acta,2010, 663, 127.

37 Y. G. Yin, M. Chen, J. F. Peng, J. F. Liu and G. B. Jiang,Talanta, 2010, 81, 1788.

This journal is © The Royal Society of Chemistry 2014

38 Z. A. Chandio, F. N. Talpur, H. Khan, H. I. Afridi,G. Q. Khaskheli and M. A. Mughal, RSC Adv., 2014, 4, 3326.

39 X. L. Song, J. H. Li, S. F. Xu, R. J. Ying, J. P. Ma, C. Y. Liao,D. Y. Liu, J. B. Yu and L. X. Chen, Talanta, 2012, 99, 75.

40 H. He, D. L. Xiao, J. He, H. Li, H. He, H. Dai and J. Peng,Analyst, 2014, 139, 2459.

41 J. E. Lofgreen and G. A. Ozin, Chem. Soc. Rev., 2014, 43, 911.42 Y. W. Liu, Y. H. Zai, X. J. Chang, Y. Guo, S. M. Meng and

F. Feng, Anal. Chim. Acta, 2006, 575, 159.43 J. J. Becker and M. R. Gagnea, Acc. Chem. Res., 2004, 37, 798.44 X. Y. Wang, Q. Kang, D. Z. Shen, Z. Zhang, J. H. Li and

L. X. Chen, Talanta, 2014, 124, 7.45 C. Herdes and L. Sarkisov, Langmuir, 2009, 25, 5352.46 J. H. Zhu, S. Y. Wei, H. B. Gu, S. B. Rapole, Q. Wang,

Z. P. Luo, N. Haldolaarachchige, D. P. Young andZ. H. Guo, Environ. Sci. Technol., 2012, 46, 977.

47 Y. S. Ho and G. McKay, Process Biochem., 1999, 34, 451.48 Y. W. Liu, X. J. Chang, D. Yang, Y. Guo and S. M. Meng, Anal.

Chim. Acta, 2005, 538, 85.

RSC Adv., 2014, 4, 46444–46453 | 46453